The so called trench MOSFETs have been developed as one type of vertical MOSFETs. The trench MOSFET includes trenches and a gate electrode buried in the trench so that channels can be formed along the side walls of the trenches. The trench MOSFET is advantageous for reducing the cell pitch and the on-resistance per unit area. Although some trench structures have been proposed and reported in the lateral MOSFET, no trench lateral MOSFET has been actually put to use in a practical power IC.
One of the trench structures proposed in a lateral MOSFET is illustrated in FIGS. 11–13, which is disclosed in Japanese Unexamined Laid Open Patent Application H09 (1997)-321291 (pp. 3 and 4, FIGS. 1–4). FIG. 11 is a top plan view of a conventional trench lateral MOSFET. FIG. 12 is a cross section taken along line XII—XII of FIG. 11. FIG. 13 is another cross section taken along line XIII—XIII of FIG. 11. The conventional trench lateral MOSFET includes a p-type silicon substrate 101, an n−-type extended drain region 103 in the surface portion of p-type substrate 101, a plurality of trenches 102 in the n−-type extended drain region 103, and a p-type diffusion region 114 surrounding each trench 102. Another p-type diffusion region 115 is in the n−type extended drain region 103. Since the p-type diffusion regions 114 and 115 form around the trenches 102, the portion of the n−type extended drain region 103, where the depletion layers expanding from the pn-junctions between the p-type substrate 101 and the n−-type extended drain region 103 and from the pn-junction between the p-type diffusion region 114 and the n−-type extended drain region 103 hardly reach, becomes depleted. Therefore, the entire n−-type extended drain region 103 is depleted easily. And, it becomes possible to expand the depletion layers long enough to relax the voltage caused between the source and the drain even when the resistance of the n−-type extended drain region 103 is set lower than that in the conventional lateral MOSFET.
In the trench lateral MOSFET exhibiting a high breakdown voltage of 50 V or higher, the drain resistance occupies a large part of the total resistance of the device. Therefore, there is a need for a semiconductor device having a structure that facilitates to reduce the on-resistance per unit area greater than conventional trench lateral MOSFETs while securing the desired breakdown voltage. The present invention addresses this need.